Superalloys are widely used as castings in the gas turbine engine industry for critical components, such as turbine blades and vanes, subjected to high temperatures and stress levels. Such critical components oftentimes are cast using well known directional solidification (DS) techniques that provide a single crystal or columnar grain microstructure to optimize properties in a particular direction.
Directional solidification casting techniques are well known wherein a nickel base superalloy remelt ingot is vacuum induction remelted in a crucible in a casting furnace and poured into a ceramic investment cluster mold disposed in the furnace having a plurality of mold cavities. The nickel base superalloy remelt ingot has been produced in the past by vacuum induction melting of elemental alloy charge components in a basic refractory lined (e.g. MgO lined) crucible or vessel. The superalloy melt is poured from the vessel onto a ceramic (e.g. alumina) tundish which includes multiple ceramic filters to remove oxide and other particles (dross) that could form inclusions in the castings.
The filtered melt is cast into steel ingot molds to produce relatively large (e.g. 100 pound) cylindrical ingots which are surface ground and end cropped or trimmed to remove pipe shrinkage at the upper end of the ingot. The ingot then is cut or sectioned to smaller remelt ingot sizes for remelting in the DS casting furnace.
During directional solidfication, the superalloy melt is subjected to unidirectional heat removal in the mold cavities to produce a columnar grain structure or single crystal in the event a crystal selector or seed crystal is incorporated in the mold cavities. Uidirectional heat removal can be effected by the well known mold withdrawal technique wherein the melt-filled cluster mold on a chill plate is withdrawn from the casting furnace at a controlled rate. Alternately, a power down technique can be employed wherein induction coils disposed about the melt-filled cluster mold on the chill plate are deenergized in controlled sequence. Regardless of the DS casting technique employed, generally unidirectional heat removal is established in the melt in the mold cavities.
Such melting and DS casting processes typcially have produced DS nickel base superalloy castings, such as high volume production turbine blade castings, having bulk sulfur impurity concentrations in the range of 2 to 10 parts per million (ppm) by weight. Such sulfur impurity levels have been thought to have an adverse effect on high temperature oxidation resistance of nickel base superalloys in service, especially as engine operating temperatures have increased.
One approach in past superalloy development to counter adverse effects of sulfur impurities on superalloy oxidation resistance has involved development of superalloy compositions modified by inclusion of a small but effective amount of an active element, such as yttrium, to improve oxidation resistance by improving stability of the alumina protective oxide that forms at the high temperatures of turbine service. Such active elements are thought to reduce the deleterious effect of sulfur impurities in the alloy composition on the protective alumina scale or layer. In particular, the presence of such active elements is observed to reduce spallation of the alumina protective layer and thus to improve oxidation resistance.
Another approach in such superalloy development has involved heat treatment of superalloy castings in a manner to reduce the sulfur concentration to low levels that provide improved oxidation resistance. For example, U.S. Pat. No. 5,346,563 describes a post-casting heat treatment for reducing sulfur levels to below 5 parts per million by weight by heat treatment at elevated temperature in the presence of MgO and other enumerated foreign chemcial species effective to alter the oxide layer to allow sulfur egress from the alloy. Examples set forth in the '563 patent involve heat treating single crystal nickel base superalloy turbine blade castings having an initial sulfur level in the range of 8 to 10 ppm by weight to achieve a reduced sulfur concentration of less than 1 ppm in the airfoil portion of castings as measured by glow discharge mass spectroscopy (GDMS) and substantilly improved oxidation resistance. However, the post-casting heat treatment involves long times (e.g. 50-100 hours) in the presence of the foreign chemical species (e.g. MgO) and thus is disadvantageous from manufacturing complexity, time and cost standpoints.
What is still needed is a method of making superalloy castings, particularly DS single crystal and columnar grain superalloy castings for gas turbine engines, having ultra low sulfur concentrations such that the cast components exhibit substantially improved oxidation resistance comparable to that of the aforementioned superalloys bearing yttrium or other active elements and the aforementioned specially heat treated superalloy castings.
What is also still needed is a method of making superalloy castings having such ultra low sulfur concentrations on a reliable, consistent basis from one casting to the next in a high volume industrial production environment or operation.